1. Field of the Invention
The present invention relates generally to the field of facsimile (fax) transmissions in real-time over a packet switching network and particularly to decreasing data traffic over the packet switching network by effectuating error-free fax transmissions over the packet switching network.
2. Description of the Prior Art
With the advent of packet switching networks, it has become efficient, pragmatic and inexpensive to transfer digital information in the form of data, audio and video, over such networks rather than the conventional public switching telephone networks (PSTN), such as conventional telephone lines. Information transmitted over packet switching networks, as the name suggests, is sent in packets. Once transmitted, these packets may be sent to devices located all over the world prior to arriving at their intended destination. The digital information that is to be transmitted is first divided into packets and transmitted over the packet switching network. Subsequently, the packets are appended reassembled to form the original digital information.
A common standard used for transmitting information over a packet switching network is the Transmission Control Protocol/Internet Protocol (TCP/IP) Reference Architecture Model. This standard includes various layers of protocol, the Internet layer of which defines an official packet format.
It has become common practice to transmit fax documents over packet switching network environment, such as the Internet. An example of such a network fax transmission is shown in FIG. 1. In FIG. 1, a prior art fax communication network 10 is shown to include a sending fax device 12, an on-ramp public switching telephone network (PSTN) 14, a sending network device 16, a packet switching network 18, a receiving network device 20, an off-ramp PSTN 22 and a receiving fax device 24. The packet switching network 18 may include many network devices, such as servers, routers and the like, for transferring information in the form of packets from one device to another located all over the world. PSTN 14 and PSTN 22 may be the same network, as conventional public telephone (telco) networks includes devices such as switches and other infrastructure used by the telephone company in routing telephone channels. The sending network device 16 is at times referred to as an “ingress gateway” and the receiving network device 20 is at times referred to as an “egress gateway”. Examples of the devices 16 and 20 are routers, access servers, and like network equipment.
In operation, the sending fax device 12, which is typically a fax machine including error correction mode (ECM) for performing correction of errors in the information being transmitted, sends a fax document that typically includes various pages, assembled in blocks, through the PSTN 14 to the network device 16. That is, each page of the fax document is scanned into the sending fax device 12 and is divided into blocks with each block including a number of frames. Each frame typically includes 256 bytes of data.
Each frame is then transmitted over the PSTN 14 to the network device 16. Each frame includes a checksum that is typically in the form of cyclic redundancy code (CRC) for use in verifying the accuracy of the frame information. The receiving fax machine 24 employs cyclic redundancy code (CRC) information to build a map of “good” frames, frames having accurate information and “bad” frames, frames having errors. The structure of the frames of a fax transmission is defined by various standards set by the industry. Examples of these fax transmission standards are T.4, T.6 and T.30. In the example of FIG. 1, frames are transmitted from the fax device 12 to the network device 16 pursuant to any of these standards.
Once the two fax machines, 12 and 24 have negotiated, a common ECM and begin to communicate through the network 18, some errors maybe incurred once fax document are transmitted over the on-ramp PSTN 14 and over the off-ramp PSTN 22 due to the presence of noise in communication lines. Additional errors may occur in the packet switching network 18 in the form of loss of packets, representing actual loss of data. Once the data arrives at the receiving fax machine 24, the receiving fax machine 24 identifies defective frames, if any, by using CRC. The receiving fax machine 24 subsequently requests that the transmitting fax machine 12 retransmit the defective frames. Upon retransmitting the defective frames, more errors may be incurred requiring further retransmission. If retransmission attempts ECM frame results in more ECM frame errors, lower speeds will be negotiated by the fax machines. The problem of transmitting and retransmitting data may continue to the point where the speed between the two fax machines 12 and 24 becomes so slow that the fax transmission begins to fail. The speed has been known to reduce to 2400 bps and finally lead to failure of the fax transmission.
There are only a certain number of times a fax message may be retransmitted. The number of times for retransmission of a fax message depends generally upon the transmitter. For example, commonly in use today, there are typically three to seven retransmission attempts, after which communication between the fax machines is terminated. Furthermore, each time fax data is retransmitted over the packet switching network 18 there is an additional contribution to the congestion in the packet switching network 18. The large number of retransmissions creates an avalanche effect on the traffic in the packet switching network 18, which could slow down the network drastically.
A main cause of congestion in the packet switching network 18, is due to the retransmission of frames that were previously in error during transmission of the fax data. Currently, there are various locations in the communications network 10 in which errors may be incurred during transmission of fax data, i.e. in the PSTN 14, PSTN 22 and the packet switching network 18. If the probability of error in the PSTN 14, PSTN 22 and in packet switching network 18 are denoted by PPSTN1, PPSTN2 and PIP, respectively, then the total probability of error in the communication system 10 is PTOT=1−(1−PPSTN1)(1−PPSTN2)(1−PIP). The formula for PTOT clearly illustrates that multiplicity of potential sources of errors is a major contributor to the overall error in the communications system 10 and consequently to the congestion of packet switching network 18.
In light of the above, the need arises to devise a method whereby ECM maybe implemented between two fax machines communicating across a packet switching network without overloading the latter. The method must be robust to correct errors incurred during transmission of data with minimum utilization of the available bandwidth in packet switching network.